Oxidative dehydrogenation process with inhibited compressor fouling



Sept. 29, 1970 D. w. MARSHALL ETAL 3,531,540

OXIDATIVE DEHYDROGENATION PROCESS WITH INHIBITED COMPRESSOR FOULING Filed Jan. 6, 1969 OXYGEN NON-CONDENSABLE GASES ORGANIC COMPOUND DEHYDROGENATION ZONE A COOLING AND CONDENSATION cOMPRESSION ZONE B ZONE c WATER SUCTION FOULING INHIBITOR W/l/uz/r/"fl. Tay/or INVENTORS l v Eli/44454 DISCHARGE 'ArraR/VA'X Patented Sept. 29, 1970 US. Cl. 260-680 Claims ABSTRACT OF THE DISCLOSURE Reducing fouling of compressor pistons and cylinders used for the compression of gaseous compositions comprising unsaturated organic compounds and carbonyl compounds by adding an alcohol or thiol into the suction side of the compressor. Preferred source of gaseous composition is from oxidative dehydrogenation process.

BACKGROUND OF THE INVENTION Field of the invention This application relates to the mechanical compression of gaseous compositions comprising unsaturated organic compounds and carbonylcompounds. The process is particularly applicable to process for the purification of gases obtained by the oxidative dehydrogenation of organic compounds such as hydrocarbons.

Description of the prior art It is known to dehydrogenate organic compounds by contacting the organic compound at an elevated temperature with oxygen, such as disclosed in US. Pats. Nos. 3,270,080, 3,303,234, 3,303,235, 3,303,236, 3,303,238, 3,308,182 through 3,308,201, 3,324,195, 3,334,152 and 3,342,890.

The dehydrogenation zone effluent from these oxidative dehydrogenation processes is purified by a process including cooling such as by quench, waste heat boilers and the like and generally the next step is to remove a major portion of the water by condensation. Thereafter the gases are compressed in a compressor. However, in these processes considerable difiiculty has been encountered due to fouling of the compressor cylinders, pistons and valves and the discharge lines from the compression. On a periodic basis it has been necessary to disassemble the compressor and clean the contact surfaces in order to prevent breakage due to fouling and plugging. Further, the operation of the compressor is less efiicient during build-up of the fouling because, for example, the valves will not properly open and close. Scoring of the piston and cylinder is also possible.

DESCRIPTION OF PREFERRED EMBODIMENTS The reason for the fouling in the compressor and in the discharge lines is not fully understood. However, it is believed that the main source of fouling is due to the presence of the various oxygenated compounds and unsaturated organic compounds in the gaseous stream being compressed.

The polymer precursors are present in the gaseous composition being compressed and in the compressed gases. These precursors may be observed as an oily liquid containing carbonyl groups which separates from the steam condensate downstream from the compressor. For example, it is conventional to have a knockout vessel downstream from the compressor to remove condensed water and these precursors may be found floating on the water condensate in the knock-out vessel. It has been discovered that polymer formation may be inhibited by mixing with these precursors an alcohol or thiol, such as methanol, which is relatively soluble in the precursors. This result is surprising in that conventional inhibitors of polymer formation such as hydroquinone did not prevent the formation of the polymer. Also ineffective as inhibitors were compounds such as glycerine, acetic acid, and triethanol amine.

A preferred embodiment of one aspect of the invention is illustrated in the drawing wherein the piston and cylinder of a reciprocating compressor is shown. The gaseous composition comprising unsaturated organic compounds contaminated with carbonyl compounds is drawn into the cylinder through the suction line. To the suction gases is added a fouling inhibitor which according to this invention is an alcohol or a thiol. The gases containing the inhibitor are then conducted through an inlet valve into the cylinder for compression.

A further preferred embodiment is illustrated in FIG. 1 of the drawing. A gaseous mixture of the compound to be dehydrogenated, air and steam are fed by line 1 to the dehydrogenation zone A. The dehydrogenation reaction may be conducted in the absence of contact catalysts, but better results are obtained if the reaction is conducted in the presence of metal or metal compound catalysts, such as disclosed in the patents cited herein. The dehydrogenation reactor may be a fixed or fluid bed reactor. The conditions of reaction may be as disclosed in any of the cited patents such as US. 3,334,152. For convenience, the invention will be illustrated for the dehydrogenation of hydrocarbons but it is understood that other dehydrogenatable organic compounds may be substituted in the example.

The efliuent 2 from the dehydrogenation zone will contain the impure unsaturated hydrocarbon products, various impurities including oxygenated hydrocarbons, noncondensable gases 1 and perhaps some unconverted hydrocarbon, oxygen and steam. When air is used as the source of oxygen, nitrogen will be present in relatively large quantities as a noncondensable gas. Steam may optionally be present in an amount up to 96 mol percent of the total effluent, such as from about 5 to 96 mol percent. The organic phase including dehydrogenated product, any unreacted feed, oxygenated hydrocarbons, poly mer and tar and precursors thereof and any organic decomposition products usually range from about 1 to mol percent of the effluent and generally will be within the range of or about 3 to 30 or 35 mol percent of the effluent. The noncondensable gases, such as nitrogen or 00 will usually be present in an amount of from or about 20 to 93 mol percent of the total etfiuent, but more The term noncondensable or inert noncondensible gases refers to those gases, other than hydrocarbons, such as nitrogen, C0 and C0, which do not condense under the conditions encountered.

often will be within the range of about 40 to 80 mol percent.

The efiluent gases 2 leaving the dehydrogenation zone will generally be at a temperature of about or greater than 600 F. or 700 F. to 1600 F. depending upon the particular dehydrogenation process. The reactor efiluent may be cooled by any means or combination of means in cooling and condensation Zone B as by quenching, waste heat boilers, condensers, vapor separators, and the like in any sequence. Preferably, the major portion of any water present in the effluent will be removed as condensed steam from the gaseous efiluent during this cooling and condensation operation. This cooled gaseous stream 3 may preferably then be compressed in compression zone C. The invention is not restricted to the particular processes prior to compression Zone C. For example, an oil quench or other step may be included.

The gaseous composition 3 to be fed to compression Zone C will usually comprise, exclusive of any Water present, about or from 3.5 to 80 mol percent of unsaturated organic compounds such as hydrocarbon, about or from 0.0005 to 2.5 mol percent of carbonyl compounds, and optionally about or from 20 to 93 mol percent of noncondensable gases (i.e., noncondensable under the conditions at point 3), all based on the total mols of gaseous composition 3 being fed to compression Zone C, exclusive of any water. Included in the noncondensable gases will be any nitrogen, oxygen, CO or CO and the like. The oxygen content may vary, but suitably will be less than 10 mol percent of 3. Steam may also be present in 3 in an amount from to 20 or up to such as 50 mol percent or more of the gaseous composition 3. Also present in the gaseous mixture 3 may be unconverted hydrocarbons such as olefins or saturated hydrocarbons and hydrocarbon by-products.

A preferred composition 3 to be fed to compression zone C will comprise, exclusive of any water present, about or from 5 to 65 mol percent of unsaturated hydrocarbons, about or from 0.005 to 1.2 mol percent of carbonyl compounds and about or from 45 to 89 mol percent of the noncondensable gases. A particularly preferred composition 3 contains about or from 8 to 65 mol percent butadiene-1,3, about or from 0.1 to 40 mol percent butene, and about or from 40 to 75 mol percent nitrogen. The composition of the compressed gases at 4 may be within the same ranges as given for point 3.

Compression in compression zone C may be by any suitable mechanical compressors such as reciprocating or centrifugal compressors, with the invention being particularly suitable for reciprocating compressors. The invention is particularly suitable for use with double acting pistons. Compressors conventionally employed in the recovery of butadiene-1,3 are suitable such as Clark reciprocating compressors. Preferred are compressors which have the cylinders cooled with a water jacket. The pressure and temperature of the gases discharging from the compressors will depend upon the particular compressor employed, the pressure and type of equipment downstream from the compressor, the temperature of cooling water available and the like, but typically will be at a temperature of at least 125 F. and a pressure of at least 75 p.s.i.g. but generally the temperature will be at least 175 F. and the pressure at least 100 p.s.i.g.

The fouling inhibitor is added to the suction side of the compressor, that is the inhibitor is added to the gases prior to or during compression. Although the inhibitor may be added directly to the cylinder, it is preferred that the inhibitor be added to the gas prior to or as the gas is drawn Except Where expressed otherwise, all references in the application are to overall quantities of carbonyl compounds as determined by ASTM Method D-lOSZ) and reported as acetaldehyde. Generally, the carbonyl compounds will have from 2 to 8 carbon atoms, e.g., from 2 to 6 carbon atoms when a C4 to Cu compound is being dehydrogenated, and will have from 1 to 2 carbonyl groups.

into the cylinder. The inhibitor may be added to the gases by any means such as by flowing from a line, through a spray nozzle including atomizing nozzles. If a spray is utilized, any type of spray may be employed, but it is a feature of this invention that an atomizing type spray is preferred. The atomization facilitates the evaporation of the water. It is also a feature of this invention that an aqueous solution of the inhibitor is used. The temperature of the gases during compression may be reduced to the addition of the inhibitor with the reduction in temperature generally being at least 15 F. and preferably the temperature will be reduced by at least 30 F. More than one spray may be employed for each cylinder. Entirely satisfactory results have been obtained by locating the spray as shown in the drawing. When the compression is conducted in several stages, the inhibitor addition is generally most effective if employed in the second or subsequent stage of compression. The usual intercoolers between stages may be employed. It is also within the scope of this invention to add the inhibitor to the compressed gases in the discharge line from the compressor.

The compressed gases may be further treated and purified as desired. The discharge gases may be treated by absorbing in an oil absorber such as described in R. C. Woerner, et al., US. 3,402,215, Sept. 17, 1968. If desired, a portion of the oil absorber overhead gases, referred to as light gases in the patent, may be used as the atomizing gases for the inhibitor spray to the comperssors. By so doing, extraneous contaminants are not introduced into the system.

The process of this invention may be applied to the recovery of products produced by the dehydrogenation of a wide variety of organic compounds. Such compounds normally will contain from 2 to 20 carbon atoms, at least one grouping, a boiling point below about 350 C., and such compounds may contain other elements, in addition to carbon and hydrogen such as oxygen, halogens, nitrogen and sulphur. Preferred are compounds having from 2 to 12 carbon atoms, and especially preferred are compounds of 3 to 6 or 8 carbon atoms.

Among the types of organic compounds which may be dehydrogenated by means of the process of this invention are nitriles, amines, alkyl halides, ethers, esters, aldehydes, ketones, alcohols, acids, alkyl aromatic compounds, alkyl heterocyclic compounds, cycloalkanes, alkanes, alkenes, and the like. Illustration of dehydrogenations include propionitrile to acrylonitrile, propionaldehyde to acrolein, ethyl chloride to vinyl chloride, methyl isobutyrate to methyl methacrylate, 2-chlorobutene-1 or 2,3 dichlorobutane to chloroprene, ethyl pyridine to vinyl pyridine, ethylbenzene to styrene, isopropylbenzene to a-methyl styrene, ethylcyclohexane to styrene, cyclohexane to benzene, methane to ethylene and acetylene, ethane to ethylene to acetylene, propane to propylene or methyl acetylene, allene, or benzene, isobutane to isobutylene, n-butane to butene and butacliene-1,3, butene to butadiene-1,3 and vinyl acetylene, methyl butene to isoprene,,

cyclopentane to cyclopentene and cyclopentadiene-1,3, noctane to ethyl benzene and ortho-Xylene, monomethylheptanes to xylenes, propane to propylene to benzene, ethyl acetate to vinyl acetate, 2,4,4-trimethylpentane to xylenes, the formation of new carbon to carbon bonds by the removal of hydrogen atoms such as the formation of a carbocyclic compound from two aliphatic hydrocarbon compounds or the formation of a dicyclic compound from a monocyclic compound having an acyclic group such as the conversion of propene to diallyl. Representative materials which are dehydrogenated by the novel process of this invention include ethyl toluene, alkyl chlorobenzenes, ethyl naphthalene, isobutyronitrile, propyl chloride, isobutyl chloride, ethyl fluoride, ethyl bromide, n-pentyl iodide, ethyl dichloride, 1,3 dichlorobutane, 1,4 dichlorobutane, the chlorofiuoroethanes, methyl pentane, methylethyl ketone, diethyl ketone, n-butyl alcohol, methyl propionate, and the like.

Suitable dehydrogenation reactions are the following: acyclic compounds having 4 to 5 non-quarternary contiguous carbon atoms to the corresponding olefins, diolefins or acetylenes having the same number of carbon atoms; aliphatic hydrocarbons having 6 to 16 carbon atoms and at least one quarternary carbon atom to aromatic compounds, such as 2,4,4-trimethylpentene-1 to a mixture of xylenes; acyclic compounds having 6 to 16 carbon atoms and no quarternary carbon atoms to aromatic compounds such as n-hexane or the n-hexenes to benzene; cycloparaffins and cycloolefins having 5 to 8 carbon atoms to the corresponding olefin, diolefin or aromatic compound, e.g., cyclohexane to cyclohexene or cyclohexadiene or benzene; aromatic compounds having 8 to 12 carbon atoms including one or two alkyl side chains of 2 to 3 carbon atoms to the corresponding aromatic with unsaturated side chain such as ethyl benzene to styrene.

The preferred compounds to be dehydrogenated are hydrocarbons with a particularly preferred class being acyclic non-quarternary hydrocarbons having 3 or 4 to 5 contiguous carbon atoms or ethyl benzene and the pre ferred products are n-butene-l or 2, butadiene-1,3, vinyl acetylene, 2-methyl-1-butene, 3-methyl-1-butene, 3- methyl-l-butene, 3-methyl-2-butene, isoprene, styrene or mixtures thereof. Especially preferred as feed are nbutene-l or 2 and the methyl butenes and mixtures thereof such as hydrocarbon mixtures containing these compounds in at least 50' mol percent.

The organic compound to be dehydrogenated is contacted with oxygen in order for the oxygen to oxidatively dehydrogenate the compound. The oxygen may be supplied to the organic compound from any suitable source as by feeding oxygen to a dehydrogenation zone for example as disclosed in US. 3,207,810 issued Sept. 21, 1965. Oxygen may be fed to the reactor as pure oxygen, as air, as oxygen-enriched air, oxygen mixed with diluents, and so forth. Oxygen may also be supplied by means of a transport or moving oxidant type of process such as disclosed in US. 3,050,572 issued Aug. 21, 1962 or US. 3,118,007 issued Jan. 14, 1964. The term oxidative dehydrogenation process when used herein means a process wherein the predominant mechanism of dehydrogenation is by the reaction of oxygen with hydrogen.

The amount of oxygen employed may vary depending upon the desired result such as conversion, selectivity and the number of hydrogen atoms being removed. Thus, to dehydrogenate butane to butene requires less oxygen than if the reaction proceeds to produce butadiene. Normally oxygen will be supplied (including all sources, e.g. air to the reactor or solid oxidant to the reactor) in the dehydrogenation zone in an amount from about 0.2 to 1.5, preferably 0.3 to 1.2, mols per mol of H being liberated from the organic compound. Ordinarily the mols of oxygen supplied will be in the range of from .2 to 2.0 mols per mol of organic compound to be dehydrogenated and for most dehydrogenations this will be within the range of .25 to 1.5 mols of oxygen per mol of organic compound.

Halogen or other additives may be present in the dehydrogenation step such as disclosed in the above cited patents, e.g., US. 3,334,152 issued Aug. 1. 1967. Means for separating halogen may also be incorporated in the dehydrogenation reactor or downstream.

Preferably, the reaction mixture contains a quantity of steam or diluent such as nitrogen with the range generally being between about 1 or 2 and 40 mols per mol of organic compound to be dehydrogenated.

The temperature for the dehydrogenation reaction generally will be at least about 250 C., such as greater than about 300 C. or 375 C., and the maximum temperature in the reactor may be about 700 C. or 800 C. or perhaps higher such as 900 C. under certain circumstances. However, excellent results are obtained within the range of or about 350 C. to 700 C., such as from or about 400 C. to or about 675 C. These temperatures are measured at the maximum temperature in the dehydrogenation zone.

The remaining conditions, catalysts, flow rates and the like for oxidative dehydrogenation are known ot those skilled in the art and may be e.g., as disclosed in US. 3,334,152 issued Aug. 1, 1967, or any of the remaining patents cited herein.

A standardized test was developed to measure the effectiveness of various compounds as inhibitors of polymer formation. According to this test, a polymer precursor was obtained by extracting some polymer which had already formed in the unit. The extract was a low molecular weight viscous oily polymer precursor. The fact that this oil was a precursor of the polymer was determined by heating the oil under controlled conditions to form additional polymer. The method for obtaining the polymer precursor is set forth in the next paragraph.

Samples of the solid polymer from the valves of a compressor were taken and five hundred grams of this solid material was extracted for one hour with 1000 cc. of petroleum ether at 23 C. The undissolved solids were removed by filtration and on subsequent evaporation of the petroleum ether extract, 3 grams of a viscous oil was recovered. Analyses of the sold residue and this viscous oil were made and results are as follows:

1 Bands characteristic of OH, carbonyls and vinyl groups.

2 Strong alkene absorbtions and carbonyls.

To further establish that this material was the precursor to the solid polymer, it was heated at 270 F. for three hours in a sealed tube. At this time the oil was observed to be polymerized into a solid with all the characteristics of the original solid resin.

In order to obtain a larger supply of the polymer precursor, additional precursor oil was obtained from the water knock-out drum downstream from the compressor. This location was chosen because the precursor tends to collect at that point as an oily layer floating in the drum. Ten gallons of the water from the knock-out drum was contacted with one-half gallon of petroleum ether. The oil that was dispersed in the water went quickly into solution in the petroleum ether and upon evaporation of the extract, 150 cc. of oil was recovered. This oil was analyzed as similar to the oil collected from the solid polymer described above.

From the following examples it may be seen that methanol, isopropanol, 2 ethyl hexanol and propane thiol inhibited polymer formation and that ethylene glycol, 1,4 butandiol, glycerine, hydroquinone, NaOH and acetic acid were not effective as inhibitors.

EXAMPLE 1 As a control run, 5 grams of the polymer precursor oil described above obtained from the knock-out drum downstream from the compressor was sealed in an cc. pressure bottle and heated for three hours at 270 F. After cooling, a heavy polymer precipitate was observed which was analogous to the original polymer from the compressor cylinders and discharge lines.

EXAMPLE 2 Example 1 was repeated with the exception that 10 per cent by weight of methanol is added to the 5 grams of polymer' precursor. The methanol was soluble in the oil under these conditions. After the same three hour test, no polymer formation was observed.

EXAMPLE 3 Example 2 was repeated with the exception that percent by weight of methanol was used, instead of percent. The methanol was soluble in the precursor. As in Example 2, no polymer formation was observed.

EXAMPLE 4 Example 2 was repeated with the exception that 10 percent by weight isopropanol is substituted for the 10 percent of methanol. The isopropanol was soluble in the precursor. No polymer formation was observed.

EXAMPLE 5 Example 2 was repeated with the exception that 10 percent by weight of 2 ethyl hexanol was used. The 2 ethyl hexanol was soluble in the precursor. As in Example 2, no polymer formation was observed.

EXAMPLE 6 Example 2 was repeated with the exception that 10 percent by weight l-propanthiol is substituted for the 10 percent of methanol. l-propanthiol was fairly soluble in the precursor. No polymer formation was observed.

EXAMPLE 7 Example 2 was repeated with the exception that 5 percent by weight of ethylene glycol was used. After the standard test, considerable polymer formation was noted.

EXAMPLE 8 Example 2 was repeated with the exception that 5 percent by weight hydroquinone is substituted for the 10 percent of methanol. Polymer formation was observed.

EXAMPLE 9 Example 2 was repeated with the exception that a 10 percent by weight sodium hydroxide solution was used instead of the methanol. Polymer formation was observed.

EXAMPLE 10 Example 2 was repeated with the exception that a 5 percent by weight acetate acid solution was substituted for the 10 percent of methanol. Polymer formation was observed.

EXAMPLE 11 Example 2 was repeated with the exception that 10 percent by weight of 1,4-butandiol was used. Polymer formation was observed. 1

EXAMPLE 12 7 Example 2 was repeated with the exception that 5 percent by weight glycerine was substituted for the 10 percent of methanol. Polymer formation was observed.

EXAMPLE 13 Reference is made to the drawing for the various pieces of equipment and streams. A hydrocarbon stream comprising butene-Z as the major component is dehydrogenated to butadiene-l,3 in reactor A. The feed 1 to the reactor includes air and steam. The eflluent 2 from the reactor comprises butadiene-l,3, unreacted butene, carbonyl compounds, steam, noncondensable gaseous components such as nitrogen and various dehydrogenation by-products such as 00 The efiiuent is cooled and most of the water is removed in the stream condensation zone B. The gaseous stream is then compressed in the compression zone C. The compressed gases at point 3 comprise by mol percent approximately a total of 64.5 percent noncondensable gases (mostly nitrogen, but also include the other residual gases of air, as well as CO and CO and 32.3 percent hydrocarbons. The hydrocarbon portion is primarily C s with butadiene-l,3 being the major component. The composition also contains 2.6 percent water and by chromatographic analysis 0.15 percent acetaldehyde, .01 percent crotonaldehyde, .05 percent acrolein and .01 percent methacrolein. The suction gases are at a temperature of F. and 35 p.s.i.g.

The compressor is operated at a discharge pressure of p.s.i.g. and temperatures of 225 F. at point 4. The compressor is handling 5780 cubic feet of gas per minute (calculated at standard conditions of atmospheric pressure and 60 F.). The addition of methanol to each cylinder is illustrated in the drawing depicting the cylinder. The methanol (fouling inhibitor) is fed at a rate of 1.0 gallon per hour per cylinder and is at a temperature of 206 F. and a pressure of 6 5 p.s.i.g. The cylinder is lubricated by oil injection directly into the cylinder and the methanol does not prevent proper functioning of this lubrication. After a period of months, the cylinders are inspected and appear in good condition.

The compressed gases may be treated to further separate and purify the gases such as by extractive distillation, CAA extraction, fractional distillation and the like.

We claim:

1. In a process for the purification of gaseous compositions comprising unsaturated organic compounds and carbonyl compounds wherein the said gaseous composition is compressed with a compressor and the compressed gases contain an oily polymer precursor containing carbonyl compounds, the improvement comprising adding to the suction side of the compressor a fouling inhibitor selected from the group consisting of alcohols, thiols and mixtures thereof which are soluble in the said polymer precursor.

2. The process of claim 1 wherein the said gaseous composition has been obtained by the oxidative dehydrogenation of a hydrocabon.

3. The process of claim 1 wherein the said fouling inhibitor is sprayed by atomizing the inhibitor with an atomizing gas.

4. The process of Claim 1 wherein the compressor is a reciprocating type compressor.

5. The process of claim 1 wherein the said fouling inhibitor is selected from the group consisting of aliphatic alcohols and thiols having from 1 to 10 carbon atoms and 1 or 2 OH or -SH groups.

6. The process of claim 1 wherein the said unsaturated organic compound is a hydrocarbon.

7. The process of claim 1 wherein the said unsaturated organic compound is butadiene-1,3.

8. The process of claim 1 wherein the said gaseous com position being compressed comprises from 3.5 to 80 mol percent of unsaturated hydrocarbon and from about 20 to 93 mol percent noncondensable gases.

9. The process of claim 1 wherein the said gaseous composition being compressed comprises from about .0005 to 2.5 mol percent carbonyl compounds.

10. The process of claim 3 wherein the said inhibitor that is sprayed into the suction side of the compressor is essentially completely vaporized prior to discharge from the compressor cylinder.

11. The process of claim 3 wherein more than one stage of compression is employed and the said inhibitor spray is sprayed into the second stage of compression.

12. The process of claim 1 wherein the compressor utilizes double acting pistons.

13. The process of claim 2 wherein the said oxidative dehydrogenation process is conducted by reacting an organic compound to be dehydrogenated with oxygen and halogen in a dehydrogenation reactor.

14. The method of claim 1 wherein the said compressor has the cylinders cooled with a water jacket.

15. The process for the compression of a mixture obtained by the oxidative dehydrogenation of a member selected from the group consisting of propane, n-butane, n-butene, n-pentane, isopentane, methyl butene and mixtures thereof to provide a gaseous mixture comprising on a Water free basis from 3.5 to 80 mol percent unsaturated hydrocarbon, from 20 to 93 mol percent noncondensable gases and from about .0005 to 2.5 mol percent carbonyl compounds, the said gaseous mixture being compressed in a reciprocating type compressor wherein the cylinders are cooled with a water jacket and wherein methanol is atomized and sprayed into the suction line to the cylinder of said compressor and thereby reducing fouling of the compressor cylinder and piston.

References Cited UNITED STATES PATENTS 3,350,282 10/1967 Davis et a1. 20354 3,371,124 2/1968 Albert et a1. 260-666.5 5 3,402,215 9/1968 Woerner et a1. 260-680 FOREIGN PATENTS 112,441 12/ 1962 Pakistan.

10 DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, Assistant Examiner US. Cl. X.R. 

